Using modern SRTM data provides an excellent and fairly quick start. Having done that its fairly easy to overlay historical maps in photoshop to adjust coastlines and river lines.
Getting the position of road and rail and urban boundaries right is not so easy due to the limitations of the game engine. The best you can aim for is just to get the right feel.
The biggest contribution to the appeal of a great map is probably the production of new artwork for ground textures and the inclusion of bump maps.
This is part of the Rabaul map by Bee. (still WIP)
